Electrographic imaging and development processes, e.g., electrophotographic imaging processes and techniques, have been extensively described in both the patent and other literature, for example, U.S. Pat. Nos. 2,221,776, issued Nov. 19, 1940; 2,277,013, issued Mar. 17, 1942; 2,297,691, issued Oct. 6, 1942; 2,357,809, issued Sept. 12, 1944; 2,551,582, issued May 8, 1951; 2,825,814, issued Mar. 4, 1958; 2,833,648, issued May 6, 1958; 3,220,324, issued Nov. 30, 1965; 3,220,831, issued Nov. 30, 1965; 3,220,833, issued Nov. 30, 1965, and many others. Generally, these processes have in common the steps of forming a latent electrostatic charge image on an insulating electrographic element, such as a photoconductive insulating layer coated on a conductive support. The electrostatic latent image is then rendered visible by a development step in which the charge image-bearing surface of the electrographic element is brought into contact with a suitable developer composition.
Many types of developer compositions, including both dry developer compositions and liquid developer compositions, have been proposed heretofore for use in the development of latent electrostatic charge images. Dry developer compositions typically suffer from the disadvantage that distribution of the dry toner powder contained therein on the surface of the electrographic element bearing the electrostatic latent image is difficult to control. These dry developers have the further disadvantage that the use thereof may create excessive amounts of dust and that high resolution is often difficult to obtain due to the generally relatively large size of the dry developer powder particles.
Many of the disadvantages accompanying the use of dry developer compositions have been avoided in the past by the use of a liquid developer of the type described, for example, in Metcalfe et al., U.S. Pat. No. 2,907,674 issued Aug. 6, 1959. Such developers usually comprise an electrically insulating liquid which serves as a carrier and which contains a stable dispersion of charged particles known as toner particles comprising a pigment such as carbon black, generally associated with a resinous binder, such as, for example, an alkyd resin. A charge control agent is often included to stabilize the magnitude and polarity of the charge on the toner particles. In some cases, the binder itself serves as a charge control agent.
To achieve suitable physical stability of the toner particles dispersed in conventional liquid electrographic developers, any of several types of various "stabilization" additives are incorporated in such a liquid developer to prevent the toner particles from settling out of the carrier liquid. Typical of such additives are those described in York, U.S. Pat. No. 2,899,335 issued Aug. 11, 1955 and various types of dispersants as described, for example, in British Pat. No. 1,065,796.
One problem which has continued to persist with conventional "stabilized" liquid electrographic developer compositions as described above is that even these "stabilized" compositions, which contain various kinds of stabilization additives, tend to become "deactivated" within a few weeks and the toner particles tend to agglomerate or settle out of the developer. As a consequence, the resultant liquid developer composition containing conventional liquid developer toner particles tends to become incapable of producing electrostatic prints of good quality and density. (See British Pat. No. 1,065,796 noted above). This deactivation of conventional stabilized liquid developers is particularly troublesome because once the toner particles settle out of the developer suspension, it is often difficult to redisperse them. And, even if redispersed, it is often found that the redispersed liquid developer does not possess the same developer characteristics as the original developer.
Developer stability, even in conventional so-called "stabilized" liquid developers has been and is still a difficult problem to overcome. The loss of "stability" which occurs in conventional liquid electrographic developers, as noted hereinabove, occurs primarily in the diluted form of the developer composition which is the "working" form of the developer, i.e., the form of developer composition actually used in most electrographic developing processes. This is one reason, in addition to convenience, that liquid developers are often prepared in the form of so-called "concentrates," i.e., mixtures of resins, pigments and/or dyes with a low liquid content. These concentrates are stable and exhibit a relatively long shelf life.
Stability in "working" liquid developer compositions may be improved to some extent, as noted above in the aforementioned York patent, by the use of various stabilization agents. These additives are most effective in a developer when used in conjunction with toner particles having a very small particle size. It has been difficult to obtain dispersing agents for liquid electrographic developers so that the dispersing agents would produce small toner particles and impart charge uniformly to all of the toner particles in the developer.
Various materials have been suggested in the prior art for dispersing pigments in liquid developers and stabilizing such developers, and such materials have found varying degrees of success. Stahly et al., U.S. Pat. No. 3,849,165 issued Nov. 19, 1974 suggest the use of copolymers having sulfoalkyl groups as dispersing agents for liquid developers. Averbach, U.S. Pat. No. 3,674,693 suggests the use of phospholipids such as lecithin as dispersing agents for liquid developers. Bivalent and trivalent metal salts of phosphorous oxyacid are suggested as dispersing agents for liquid developers in Gilliams et al., U.S. published patent application No. B376,749 published Mar. 30, 1976, and in British Pat. Nos. 1,151,141; 1,411,287; 1,411,537; and 1,411,739.
There is a continuing need to find new and improved dispersing agents that will produce stable, small toner particles having a uniform charge.